Conventional gamma ray detectors to be used as survey meters and monitoring posts have been configured to determine a count rate, i.e., a count per unit time or a dose rate, i.e., a dose per unit time.
Then, in order to measure such important information as the incoming direction of radioactive rays, a method has been adopted in which a gamma ray detector is equipped with a large heavy collimator made of lead that it has sensitivity only to gamma rays of one direction passed through the lead collimator. This method, however, has had the problem that the heavy load of the lead collimator makes the detector greater in size and impairs portability thereof. Moreover, there has been another problem that since gamma rays are incident on the detector only in some directions, it is impossible to determine the count rate and dose rate of the radioactive rays (incident from all directions) in that site.
In order to solve these problems, as described in Japanese Patent Laid-Open Publication No. Hei 5-66275(Patent Document 1), for example, there has also been developed a radiation detector 30 in which a plate-like thin plastic scintillator 32 of 5 mm or so and a CsI(Tl) scintillator 34 are joined in front and behind as shown in FIG. 1, so that directivity occurs from the relationship between the intensities and the characteristics in shape, such as a rise and a fall, of electric pulses that are generated by the incidence of gamma rays. In FIG. 3, the reference numeral 36 is a photoelectric transducer device, and 38 is a light-shielding case.
Nevertheless, (1) the plastic scintillator 32 has a low density, and the probability for the plastic scintillator 32 to react with, for example, 662-keV gamma rays of Cs137 incident from the front is 5% or so at best, based on calculations using the public code EGS4 of Monte Carlo simulation method. Ninety-five percent enters the subsequent CsI(Tl) scintillator 34 without reaction, which in principle makes calculation impossible. That is, there occurs the problem of low detection efficiency of gamma rays. Moreover, (2) the response sensitivity varies greatly with the angle of incidence. That is, the sensitivity is high in the forward direction to the plastic scintillator 32, whereas the sensitivity drops sharply in lateral directions beyond 20°. The detector is thus unsuitable to monitoring posts or the like which require a wide directivity. Besides, (3) setting the directivity requires an additional operation of recognizing complicated factors such as the intensities and the shapes of electric pulses. Moreover, (4) the detector has no sensitivity backward. Furthermore, there has also been the problem that (5) it is impossible to determine the count rate and dose rate of radioactive rays in that site.